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Prediction of Initial Thyroid Therapy Clinical Exposure Rates Given the Amount of I-131 Activity and the Patient Habitus

L Williams

C Pickering, J Dykes, J Mas, M Domingo, D Yamauchi, G Lopatin, J Patricko, L Williams*, City of Hope Medical Center, Duarte, CA

SU-E-T-288 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Thyroid cancer (papillary and follicular) patients were evaluated for initial external radiation exposure rate (r) at the time of ingestion of their I-131 capsule. It is important to measure this rate (mR/h) and establish its variation with individual physical parameters such as age, sex, height, weight and body mass index (BMI). In this way, staff and public exposures can be estimated a priori for any future I-131 therapy given the parameters of the prospective patient. Methods: Over the 2006 to 2010 interval, 156 consecutive individuals were measured for r values. A calibrated ion chamber was used to determine the initial exposure rate at one meter from the navel. Because of HIPPA requirements, only 61 of these patients could have their medical records reviewed for personal information. For this subset of individuals, correlations were made of r/activity with age, sex, height, body mass and body mass index (BMI). Results: The average female initial exposure rate was 0.150 +/- 0.044 mR/(h*mCi) with the male value being 0.154 +/- 0.033 mR/(h*mCi). There was no significant correlation on initial rate/activity with age, sex or height. Significant negative correlation of r/activity was found (p < 0.05) for patient weight (-0.296) and BMI (-0.386). In the former case, the probability was 0.021 and in the latter 0.00214 that the correlation was due to random causes. Conclusions: Mean initial exposure rate/activity for male (24) and female (37) patients were not significantly different. Largest correlation of r/activity was seen with the BMI index with weight being the second strongest correlation. Average patient initial exposure rate/activity at 1 meter decreased from 0.17 mR/(h*mCi) to 0.13 mR/(h*mCi) as BMI increased from 20 to 40 Kg/(m*m). BMI has significant effect on clinical exposure rates in thyroid cancer therapy.

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